1. Field of the Invention
The present invention concerns a suspension rod tensioning arrangement of the type used to support cryogenic equipment within a cryostat.
2. Description of the Prior Art
In order to improve access for clinicians and improve patient comfort, it is advantageous to make magnets used in MRI systems as compact as possible. The superconducting magnet typically used must be held at cryogenic temperatures, and this requires that superconducting magnets must be suspended by a suspension system, and heat conduction through the suspension system must be minimized.
Magnetic resonance imaging (MRI) systems typically include a cryogenically cooled superconducting magnet, retained within a cryostat, essentially comprising an evacuated outer vacuum vessel. Suspension arrangements are typically provided, and comprise a number of support rods or bands or similar. The suspension arrangements are designed to have very high tensile strength, to withstand both the static load of the weight of the magnet and its cooling arrangements, and dynamic loads resulting from operation of the MRI system, or transportation of the magnet in the cryostat.
Suspension rods of high strength metal such as austenitic stainless steel are an established way of supporting cryogenically cooled superconducting magnets within cryostats. FIG. 5 schematically illustrates a conventional support arrangement for a cryogenically cooled magnet within a cryostat. Feature 52 represents a superconducting magnet with its cooling arrangement. As is conventional, this may include a number of coils mounted on a former housed within a cryogen vessel partially filled with liquid helium. However, other cooling arrangements may be provided and the present invention is not limited by the particular cooling arrangement chosen. The magnet and cooling arrangement, 52, is housed within an outer vacuum vessel 54. This provides thermal isolation from ambient temperature. Other features such as thermal radiation shields may be provided, as is known to those skilled in the art, but are not shown in FIG. 5 for clarity. The magnet and cooling arrangement 52 is retained by a number of support rods 10. As illustrated in the drawing, it is conventional to try to have these rods as long as possible, so as to increase their thermal resistance and so reduce the thermal influx through the material of the support rods 10 to the magnet and cooling arrangement 52. Support brackets are provided on the magnet and cooling arrangement 52 and the outer vacuum vessel 54. The support rods 10 are tensioned between the brackets to bear the static and dynamic loads caused by the magnet and cooling arrangement 52. The notion of thermal length represents the distance through which conducted heat has to travel from the outer vacuum vessel 54 to reach the magnet and cooling arrangement 52. The cross-section and thermal conductivity of parts of the heat path may also be taken into account when considering the thermal length. Thus, while principally concerned with length, the concept of thermal length represents thermal resistance between two objects.
In conventional designs thermal load is minimized by the use of long rods, typically with threaded ends to facilitate assembly and tensioning. However, these designs result in relatively large cryostats to provide space for assembly and suspension rod termination systems. In order to reduce the required length, high strength materials with lower conductivity than metals may be used, for example glass or carbon fiber composites, but these are inherently expensive to manufacture.